1. Field of the Invention
The present invention relates to a hydraulic pressure control system for an automotive vehicle, and more particularly to the system having a dual circuit brake system.
2. Description of the Related Arts
As a hydraulic braking pressure control system for an automotive vehicle, the one with an anti-skid control function for decreasing, increasing or holding a braking pressure to each wheel cylinder to obtain a maximum coefficient of friction, has come into wide use. For example, Japanese Utility model publication No.6-35842 discloses such a hydraulic braking pressure control system having (a) a hydraulic pressure supply apparatus, (b) an electromagnetic hydraulic pressure control apparatus which controls a hydraulic pressure of a hydraulic pressure source to produce a second output hydraulic pressure, and supply the second output hydraulic pressure to a second wheel brake cylinder through a second hydraulic passage, (c) a transmission changeover apparatus which is provided in the second hydraulic passage, to connect a pressure chamber which produces a first output hydraulic pressure of a hydraulic pressure supply apparatus even in the case where leakage of fluid is caused in a portion for holding a pressure from the electromagnetic hydraulic pressure control apparatus, and which transmits the second output hydraulic pressure to the second wheel brake cylinder of the electromagnetic hydraulic pressure control apparatus, but transmits the first output hydraulic pressure of the hydraulic pressure supply apparatus when the magnetic hydraulic pressure control apparatus broke down, and (d) shutting means provided in the transmission changeover apparatus for shutting off the flow of fluid from the transmission changeover apparatus to at least either one of the electromagnetic hydraulic pressure control apparatus and the second wheel brake cylinder, in the case where the fluid is caused to be leaked from the hydraulic pressure supply apparatus due to a trouble of either one of them.
In the system as disclosed in the above Japanese Utility model publication No.6-35842, a hydraulic braking pressure condition is detected by a hydraulic pressure sensor (98, 100 in FIG. 1 of the publication), so that malfunction of hydraulic circuit is determined on the basis of the detected result. However, these hydraulic pressure sensors are high in price, so that it has been desired to constitute an apparatus which never requires any hydraulic pressure sensors including sensors provided for hydraulic pressure control.